Numerous African non-human primates, including the sooty mangabeys, are natural hosts of SIV and have co- evolved with their respective SIVs for at least 30,000 years. Studies of SIV infections in natural host species have dramatically influenced the way we think about AIDS pathogenesis in two fundamental ways. First, prevention of disease progression does not require suppression of virus replication, as natural SIV hosts remain free of clinical signs despite high levels of viremia. Second, the absence of AIDS correlates with low levels of systemic immune activation during chronic infection. The Sodora laboratory has established a cohort of SIV-infected mangabeys that unexpectedly developed severe CD4+ T cell depletion (CD4 blood levels are <80 cells/ul) but still do not progress to AIDS. These CD4-low animals challenge the current paradigm that very low CD4+ T cell counts are not compatible with long term survival. The key unanswered question that the SIV-infected CD4-low mangabey cohort invokes is: How do these sooty mangabeys maintain ostensibly normal T helper cell function with so few CD4+ T cells? In this proposal we will test the hypothesis that CD3+CD4-CD8- double-negative (DN) T cells provide basic T helper cell functions in CD4-low SIV-infected sooty mangabeys, allowing for maintenance of a normal immune system, whereas DN T cell function is sub- optimal during the CD4+ T cell depletion that follows pathogenic HIV/SIV infections in humans/macaques. DN T cells are inherently resistant to SIV infection due to the absence of the CD4 receptor. In addition to studying the function of these cells in sooty mangabeys, we will investigate the potential for DN T cells to function as helper cells during pathogenic HIV and SIV infections of humans and macaques. In previous studies, we have demonstrated that in CD4-low SIV-infected sooty mangabeys, DN T cells: (i) Are present at relatively high levels in peripheral blood both prior to and following an SIV infection; (ii) Can be SIV specific and produce cytokines in response to SIV peptide stimulation; (iii) Principally exhibit a central memory T cell phenotype; and (iv) Produce Th1, Th2 and Th17 cytokines. These preliminary data indicate that the DN T cells provide helper T cell like function that could assist in immune responses against both SIV and opportunistic pathogens in CD4-low mangabeys. The specific aims of the proposal will assess phenotype and function of DN T cells in SIV-infected mangabeys as well as in two pathogenic infections, SIV-infected macaques and HIV infected humans (Aim 1), as well as the ability of DN T cells to make antigen specific responses in immunized mangabeys (Aim 2). Ultimately, this work will provide the basic evaluations necessary to determine if a future immune therapeutic intervention to increase function of DN T cells during pathogenic HIV/SIV infections is possible. We envision that immune therapeutic approaches improving DN T cell function may provide a 'Functional Cure' in which HIV-infected individuals will achieve a non-pathogenic state that is similar to what we have documented in SIV-infected natural hosts.